THE WORD EPIDEMIOLOGY is derived from the Greek epi (“upon”), demos (“people”), and logos (“study of”). It refers to the study of phenomena that have been put “upon” people, specifically, afflictions that weigh upon the health of a population over space and time. Epidemiology is a multidimensional science that deals with the study and occurrence of diseases, attempting to answer many questions: the origin, causes, loci, and spatial distribution of diseases and how these aspects have changed over time. To answer these questions, epidemiology often takes a historical perspective and borrows from the fields of medicine, ecology, anthropology, sociology, and demography. Geography is involved because the spread and spatial patterns of diseases are analyzed.
Epidemiology in the Western world can be traced back to the ancient Greek civilization, where Hippocrates (460–388 B.C.E.) laid the foundations for modern epidemiology through observation and description of diseases in relation to the environment. Sporadic references to epidemiology occur throughout history, such as references to links between illnesses and environment, and between public health and sanitation. Scientific inquiry gained ground by the 17th century, and like other sciences, epidemiology has shifted its theoretical allegiance many times since.
Early epidemiological studies were descriptive in nature, detailing disease symptoms, areas of influence and affected population groups and subgroups. Causes were often speculated upon based on the reigning belief structure. In the 18th century, cause was often based on the miasma theory (diseases borne by air). The 19th century saw the popularity of the germ theory, which focused on the role of microorganisms in causing and spreading disease. Currently, epidemiologists believe that a number of factors (for example, environment, microorganisms, genetics) play into creating diverse disease patterns.
John Snow, a 19th-century London physician, performed one of the first formal epidemiological studies in modern history, which has since become a legend. London had been ravaged by cholera, but the cause was yet unknown. As for all contagion in the day, miasmas were believed to be the culprit, but Snow suggested that the disease spread through contamination of food and water by way of the water supply. He was finally able to validate his theory during London's 1854 epidemic by mapping the locations of cholera deaths and of water pumps in the city. He found that the Broad Street Pump showed the greatest concentration of deaths near it. He suggested the removal of its handle, which, incredulously for the townspeople at the time, resulted in a containment of the epidemic. This exercise demonstrated the importance of sanitation and prevention measures in the containment of diseases and epidemics, a lesson used even today.
It would be erroneous to believe that epidemiology is a Western science. “Non-Western” or traditional civilizations present evidence of epidemiological studies that predate even the work of the ancient Greeks. Careful observation of symptoms and possible causes had been noted in the civilizations of EGYPT, CHINA, INDIA, and the Americas. Treatises such as Charaka Samhita by the Indian physician Charaka, and Zhubing Yuan Hou Lun (On Pathogenesis and Manifestations of All Diseases) by the Chinese physician Chao Yuanfang were written, which contributed greatly to the development of epidemiological studies and the promotion of public health. Sadly, much of this was lost, downplayed, or overshadowed by Western knowledge and paradigms.
Some of the benefits derived from epidemiology are a better understanding of the causes and etiology behind illnesses and health problems; the impact of changing technologies, environments, and lifestyles on our health; the ability to project longevity and morbidity patterns on the basis of this understanding; the ability to provide prognoses and generalizations; and finally, the provision of solutions through prevention and public health measures.
These uses are evident in the case of AIDS (acquired immunodeficiency syndrome), which was first identified only in isolated cases in the early 1970s but has now become a pandemic (global epidemic). Through epidemiological studies, it is now known that AIDS is primarily caused by the human immunodeficiency virus (HIV). Two strains of this virus exist, one originating in Central Africa and the other in West Africa. Both have diffused over travel routes to other parts of the world.
Epidemiology has revealed the nature, origin, diffusion routes, and modes of transmission (blood and blood product transfusions, sexual contact, needle sharing, and mother-to-child) of AIDS. Early identification of symptoms and risk factors, more accurate prognosis, and identification of geographic patterns of occurrence are instrumental in containing its rapid spread and in formulating effective health programs.
The descriptive style of traditional epidemiology has been replaced by a more complex set of theories and methods comprising the current modern approach, which uses statistical techniques and models. However, epidemiology's status as a true science is often questioned on the grounds that it lacks a theoretical matrix. However, epidemiology does claim underpinnings such as the “agent-host-environment” triad and “iceberg” concept. “Iceberg” is an analogy that describes a widely occurring phenomenon in healthcare, that of undiagnosed or misdiagnosed cases being hidden from view, much like the unseen portion of an iceberg. These theories are subject to the rigors of testability through the use of appropriate measures and tools. Some commonly used methods and tools are cohorts, case-control studies, and rates and ratios.
As epidemiology advanced, it became apparent that diseases were affected by geography, in terms of both absolute location and what was contained within that location. This realization of areal variation of diseases contributed to the legitimization of medical geography as a field of study in its own right.
Two approaches evolved: disease ecology and disease diffusion. Disease ecology deals with the interplay among environmental conditions (physical and social), the human host, and the agent of the disease. Disease diffusion examines the spread of diseases from their points of origin. An integrated approach is probably the best means to a complete “picture of health.”